The present invention relates to a method of manufacturing diffractive optical elements and more particularly to a method of manufacturing diffractive optical elements using photolithographic mastering and audio/video disc manufacturing equipment and processes.
Diffractive lens elements have been made by photolithographic manufacturing techniques. First, a pattern is produced by an optical designer with appropriate output file formats to be written by e-beam into a single or series of photomask(s). The patterns may have a distinct binary or multiphase grating designed to create a desired diffraction effect. Photolithographic processes are then used to transfer the pattern in the photomask(s) into a substrate having the necessary mechanical and transmissive characteristics. The substrate can be quartz, fused silica or other material.
It has also been suggested that diffractive lens elements or multiple diffractive and refractive integrated optical assemblies can be formed by plastic replication techniques. Photomasks are produced and used to create a master for molding. The mold materials must be durable enough to withstand the molding process. The diffractive patterns are transferred to the mold master using photolithographic processes specifically characterized for the physical configuration and material being used. U.S. Pat. No. 5,538,674 to Nisper et al., the disclosure of which is incorporated herein by reference, illustrates a method of making holograms, kinoforms, diffractive optical elements and microstructures. U.S. Pat. No. 5,013,494 to Kubo et al., the disclosure of which is incorporated herein by reference, illustrates a method of making desired surfaces using injection mold techniques.
Prior art systems for producing plastic diffractive lens elements or lens systems have a number of disadvantages. The molds are usually single purpose tools dictated by the physical size of the diffractive lens. Since each mold is designed for a specific application, a manufacturer may incur significant costs to justify tooling. In many cases, the projected volume of the product being produced will not justify the cost.
Thus, alternate manufacturing processes are used, such as straight etching of the desired pattern into a substrate which is then cut to the desired form factors. In order to maintain the maximum efficiency of the diffractive lens, multiple phase steps are required by the design. In manufacturing, this requires the initial etching of the pattern using photolithographic processes and subsequent mask alignment or multiple mask alignment to the previous etch or etches. This process is both time consuming and costly.
Moreover, even in cases where the production volume justifies the expense to produce a mold base, there is the disadvantage that the system can produce only one optical element or lens system per molding operation. An additional disadvantage is the production lead time required. The production lead time may exceed six months for the design and construction of the mold.
Moreover, custom tooling and refined characterization of the photolithographic procedures may be required. In addition, significant time may be required to characterize both the new mold and the molding process for the specific application. Even during production, the throughput or capacity of the mold is often limited.
In cases where a mold is xe2x80x9creusedxe2x80x9d for multiple products, the generic mold base must be fitted with diffractive pins customized for the application. These pins must be fabricated and then etched with the desired patterns. This may require weeks of tooling to complete.
The present invention alleviates to a great extent the disadvantages of the prior art by using existing audio/video disc manufacturing processes and equipment to create diffractive optical elements.
In one aspect of the invention, audio/video manufacturing equipment is modified and used to manufacture an array of diffractive optical elements.
It is an object of the present invention to reduce the time required to realize end products from design inputs. With the present invention, once photomasks are produced for the designed optic, mold mastering may be accomplished in a few days. Once a plate with the desired surface relief pattern is completed, plastic products can be produced in a few hours. Thus, the time it takes to produce a new product is reduced.
It is another object of the present invention to utilize the existing capital equipment base residing in the compact disc industry. U.S. Pat. No. 4,185,955 to Holmes et al. and U.S. Pat. No. 4,707,321 to Segawa, the disclosures of which are incorporated herein by reference, illustrate systems for molding centrally apertured video disc records. The molds used by the invention reside in large numbers in industry. As new technology emerges for mass storage of digital data, these molds will become even more accessible to the optics industry. Diffractive optics being produced by these techniques have recurring costs an order of magnitude less than those being produced using other technology.
It is another object of the present invention to use pre-existing photolithographic masks to produce molds used with audio/video compact disc manufacturing equipment.
It is another object of the present invention to produce high quality optical products. Manufacturing process used by the compact disc industry are well documented and defined by compact disc manufacturers. Molding characteristics of polycarbonate for compact discs is also well understood by those in the compact disc industry. Thus, compact disc manufacturing equipment can be used in the present invention to produce high quality optical elements.
It is another object of the present invention to create diffractive optics in an essentially planar array. This allows large elements to be stacked together to correct optical aberrations much in the same way as conventional glass lenses may be aligned to correct for chromatic and spherical aberrations.
Creating diffractive optics in an essentially circular planar array also allows stacked elements to be rotated with respect to each other about their central axis. This allows variability in polarization, beam scanning, and wavelength selection to be accomplished.
It is another object of the present invention to provide industry with a low cost, high production volume process for the replication of diffractive optics.
Other objects and advantages of the invention will be readily apparent from the following description and drawings which illustrate preferred embodiments of the invention.